1. Field of the Invention
The present invention relates to resistance measurement. More specifically, the present invention is directed to a measuring circuit for measuring the difference in resistance between two resistors.
2. Description of the Prior Art
The measurement of an unknown parameter by the use of a parameter responsive resistance is a technique used in many applications, e.g., temperature measurement, using resistance temperature bulbs. Accordingly, it is desirable to provide a direct method of measuring the resistance of a resistive element which may, as in the case of a resistance temperature bulb, be representative of the temperature detected by the resistive element. Typically, the prior art circuits for measuring such resistances have included a bridge circuit such as that shown in U.S. Pat. No. 3,817,104. Such a prior art bridge circuit while providing for a measurement of the resistance change of the unknown resistor produced an output voltage which was a non-linear function of the unknown resistance variations. Another prior art circuit for measuring such resistance variations has involved the use of two constant current power supplies for energizing the unknown resistor as shown in U.S. Pat. No. 3,913,403. This prior art technique while eliminating the non-linearity problem of the aforesaid bridge circuit introduced a new problem by requiring a precise matching and tracking of the two constant current sources. Further, neither of these prior art techniques afforded a convenient method for providing "zero suppression" of the output signal to produce a zero output signal at a desired resistance value of the measured resistance. Accordingly, it is desirable to provide a resistance measuring circuit for measuring resistance variations while providing a linear output and zero suppression while using only one constant current source.